I think the biggest argument against downvotes of a proposal is that it gives more power to large stakeholders to downvote a proposal they don't like, without interfering with other proposals. Right now, if I wanted to defund a proposal that was voted in, my only recourse would be to raise the refund limit (assuming I wasn't already voting on it with my full strength), potentially cutting off proposals I want to get funded. With downvoting, that would not be true.

So it really depends on whether stakeholders want more or less proposals to get funded I guess (I'm not really sure that is how it would work out, but that's my best guess).

Might be more of a threat in the future when cryptocurrency is worth more, and people convert from major coins into Hive, for example. Then, maybe someone comes over with a lot of power and upvotes their own proposal to give themselves 50k HBD a day for a year and just dumps it.

I don't think this is much of a threat, because to do it they would have to such a huge stake and lock it up, and then they have to worry about being forked out by everyone else. If they could achieve a proposal attack on the DHF, they can do the same thing to witness votes, but we saw how that worked out for Justin Sun.